Femoral nerve block

Surgical approach (where this block is used)

• Not an operation itself; commonly used as analgesia/anaesthesia for surgery involving the anterior thigh and knee.
• Typical operations where femoral nerve block (FNB) may be requested/used
  • Femoral shaft fracture fixation / traction / splintage (analgesia in ED/trauma)
  • Knee surgery: arthroscopy, ACL reconstruction (often combined with other blocks), patella surgery
  • Total knee arthroplasty (increasingly adductor canal block preferred for motor-sparing; FNB still relevant)
  • Skin grafting / wound care to anterior thigh

Anaesthetic management (typical context)

• Type of anaesthesia
  • Regional analgesia adjunct to GA for knee/thigh surgery; or standalone block for analgesia (e.g., femoral fracture) with/without light sedation.
  • If used as sole anaesthetic for surgery: usually requires additional blocks (e.g., sciatic/obturator/lateral femoral cutaneous) depending on procedure.
• Airway
  • If GA: SGA often suitable for short knee arthroscopy; ETT for longer/complex surgery, aspiration risk, positioning, or need for paralysis.
  • If block + sedation: maintain spontaneous ventilation; be prepared to convert to GA if inadequate block or agitation/pain.
• Duration
  • Block performance: typically 5–15 min; onset 10–30 min (agent dependent).
  • Analgesia duration: ~6–18 h (single-shot; depends on LA choice/adjuncts). Continuous catheter can provide days.
• How painful is the surgery?
  • Femoral fracture and knee surgery can be very painful; multimodal analgesia usually required.
  • FNB covers anterior knee/anterior thigh well but does not reliably cover posterior knee pain (often sciatic contribution).
• Key perioperative considerations
  • Motor weakness (quadriceps) → falls risk; mobilise with caution and document advice.
  • Anticoagulation status and infection risk (particularly for catheter techniques).
  • Local anaesthetic systemic toxicity (LAST) risk: calculate maximum dose; incremental injection with aspiration; consider ultrasound guidance.

Overview

• Femoral nerve block provides analgesia/anaesthesia to the anterior thigh and much of the anterior knee; also contributes to medial leg/foot via saphenous nerve (terminal sensory branch).
• Common uses: analgesia for femoral fractures; perioperative analgesia for knee surgery; can be part of combined blocks for more extensive lower limb surgery.

Applied anatomy (high yield)

• Origin and course
  • Lumbar plexus: posterior divisions of L2–L4 (sometimes L1 contribution).
  • Emerges lateral to psoas major, runs between psoas and iliacus, passes under inguinal ligament into femoral triangle.
• Relations in femoral triangle (classic exam anatomy)
  • Lateral to femoral artery; outside femoral sheath (artery/vein within sheath; nerve outside).
  • Order lateral → medial: femoral Nerve, Artery, Vein, (empty space), Lymphatics ("NAVeL").
  • Often lies beneath fascia iliaca; fascia lata is more superficial.
• Branches and sensory/motor supply (what it covers)
  • Motor: quadriceps femoris (knee extension), sartorius, pectineus (variable).
  • Sensory: anterior thigh (anterior cutaneous branches); medial leg/ankle/foot via saphenous nerve; articular branches to hip and knee.
  • Does NOT reliably cover: lateral thigh (lateral femoral cutaneous nerve), posterior knee (sciatic), medial thigh (obturator).

Indications

• Analgesia for femoral shaft/neck fractures (often as part of fascia iliaca compartment block pathway; FNB is a targeted alternative).
• Perioperative analgesia for knee surgery (arthroscopy, ligament reconstruction, patella surgery, TKA—though adductor canal block often preferred for mobilisation).
• Analgesia for anterior thigh procedures (skin graft donor site).
• Diagnostic/therapeutic pain interventions (less common in FRCA context).

Contraindications

• Absolute
  • Patient refusal / lack of capacity without appropriate best-interest decision.
  • Allergy to local anaesthetic (true IgE-mediated rare; clarify reaction).
  • Infection at injection site.
• Relative / caution
  • Anticoagulation/bleeding diathesis: peripheral nerve blocks generally lower risk than neuraxial, but femoral region is non-compressible deep to fascia; consider haematoma risk and local policy/ASRA/RA-UK guidance.
  • Pre-existing femoral neuropathy or significant peripheral neuropathy (risk of attribution/worsening).
  • Severe sepsis/haemodynamic instability (risk–benefit; may still be appropriate for analgesia but ensure resuscitation and monitoring).
  • Inability to cooperate/position safely (consider sedation/GA or alternative technique).

Preparation and monitoring (OSCE/viva structure)

• Consent: purpose, expected benefits, alternatives (systemic opioids, fascia iliaca block, adductor canal block), and specific risks.
• Monitoring: standard AAGBI monitoring; IV access; resuscitation equipment and intralipid immediately available.
• Asepsis: full barrier precautions for catheter; at least sterile gloves, skin prep, sterile probe cover/gel for ultrasound.
• Local anaesthetic safety: calculate maximum safe dose (consider lean body weight, age, frailty); use incremental injection with frequent aspiration; consider using lower concentration/volume when appropriate.

Techniques

• Approaches
  • Ultrasound-guided (preferred): direct visualisation of nerve, artery, fascia; lower vascular puncture and potentially lower LA volume.
  • Nerve stimulator (landmark-based): relies on motor response (quadriceps/patellar twitch).
  • Landmark/paresthesia techniques: less favoured due to higher failure/complication risk.
• Ultrasound anatomy (transverse at inguinal crease)
  • Femoral artery: pulsatile anechoic circle; femoral vein medial and compressible.
  • Femoral nerve: hyperechoic, triangular/oval structure lateral to artery, under fascia iliaca, superficial to iliopsoas.
  • Fascia lata (superficial) and fascia iliaca (deeper) appear as hyperechoic lines; aim to deposit LA under fascia iliaca around nerve.
• Ultrasound-guided steps (in-plane lateral-to-medial typical)
  • Position: supine, leg slightly abducted and externally rotated; expose groin; identify landmarks and scan.
  • Needle: in-plane, advance towards lateral aspect of nerve; avoid intraneural injection (high resistance, nerve swelling, pain/paresthesia).
  • Hydrodissection with small aliquots to confirm correct plane under fascia iliaca; then inject incrementally to surround nerve (circumferential spread desirable but not always necessary).
  • Aspirate every 3–5 mL; observe spread; reassess needle tip frequently.
• Nerve stimulator technique (key points)
  • Needle inserted 1–2 cm lateral to femoral artery at inguinal crease; seek quadriceps contraction/patellar twitch.
  • Typical stimulator settings: start 1.0 mA, 0.1 ms, 2 Hz; acceptable response at ~0.2–0.5 mA suggests close proximity (avoid <0.2 mA with strong response—possible intraneural).
  • Inject after negative aspiration; stop if severe pain/paresthesia or high resistance.
• Catheter technique (continuous femoral nerve block)
  • Indications: major knee surgery (e.g., TKA) where prolonged analgesia desired; consider motor weakness vs adductor canal catheter alternative.
  • Thread catheter 3–5 cm beyond needle tip under ultrasound; secure well; label catheter; prescribe infusion with clear limits and monitoring plan.

Local anaesthetic choice and dosing (typical ranges; tailor to patient)

• Single-shot volumes commonly 10–20 mL with ultrasound (higher volumes may be used with landmark techniques but increase LAST risk).
• Agents
  • Lidocaine 1–2%: faster onset, shorter duration.
  • Bupivacaine 0.25–0.5% or levobupivacaine 0.25–0.5%: longer duration.
  • Ropivacaine 0.2–0.5%: long-acting with relatively less motor block at lower concentrations (still causes quadriceps weakness with FNB).
• Adjuncts (departmental policy dependent)
  • Dexamethasone (perineural or IV) may prolong duration; consider infection risk, hyperglycaemia, and local governance.
  • Clonidine may prolong block but can cause hypotension/sedation.

Block assessment and expected effects

• Sensory: reduced cold/pinprick anterior thigh and medial leg (saphenous).
• Motor: reduced knee extension (quadriceps) and reduced patellar reflex.
• Analgesic limitations: persistent posterior knee pain suggests sciatic contribution; lateral thigh pain suggests lateral femoral cutaneous nerve; medial thigh pain suggests obturator.

Complications and management

• Block failure / incomplete block
  • Causes: wrong plane (above fascia iliaca), inadequate volume, anatomical variation, intravascular uptake, time insufficient for onset.
  • Management: reassess clinically and with ultrasound; consider top-up/repeat with safe dosing; supplement with additional blocks or systemic analgesia; convert to GA if required.
• Vascular puncture / haematoma
  • Avoid: ultrasound identification of artery/vein, in-plane visualisation, aspiration, gentle technique.
  • Manage: direct pressure, reassess anticoagulation, document; consider imaging/surgical review if expanding haematoma or neurovascular compromise.
• Local anaesthetic systemic toxicity (LAST)
  • Early features: tinnitus, metallic taste, circumoral numbness, agitation, seizures; later: arrhythmias, cardiovascular collapse.
  • Immediate management: stop injection, call for help, airway/oxygen/ventilation, treat seizures (benzodiazepine), start intralipid per guidelines, manage arrhythmias (avoid large doses of propofol in unstable patient; avoid vasopressin; use reduced-dose adrenaline).
• Nerve injury
  • Mechanisms: intraneural injection, needle trauma, ischaemia/haematoma, neurotoxicity, stretch/compression.
  • Prevention: avoid paresthesia/pain on injection, avoid high opening pressure, keep needle tip in view, use minimal effective dose, avoid deep sedation during injection so patient can report symptoms.
  • If suspected: stop, document, examine and follow local nerve injury pathway; early neurology referral if severe/progressive deficit.
• Infection (esp. catheter)
  • Asepsis, secure dressing, daily review; remove catheter if signs of infection or unexplained sepsis.
• Falls due to quadriceps weakness
  • Warn patient and staff; physiotherapy precautions; consider knee brace if mobilising; consider adductor canal block as alternative for motor-sparing knee analgesia.

Comparisons and related blocks (common viva angles)

• Femoral nerve block vs fascia iliaca compartment block (FICB)
  • FNB: targeted block of femoral nerve; reliable quadriceps weakness; good anterior thigh/knee analgesia.
  • FICB: aims to block femoral + lateral femoral cutaneous ± obturator by high-volume spread under fascia iliaca; often used in ED/trauma pathways; may be less reliable for obturator.
• Femoral nerve block vs adductor canal block (ACB)
  • ACB targets saphenous nerve/nerve to vastus medialis in adductor canal → better motor preservation for mobilisation after knee surgery, but may provide less analgesia than FNB for some procedures.

Documentation (often examined in OSCE)

• Record: indication, side, technique (US/NS), asepsis, LA drug/concentration/volume, adjuncts, needle/catheter details, complications, block assessment, advice re falls, and post-block monitoring plan.